Retardation kinetics

In addition, the effects of gas phase retardants can change both A and E. If E is increased, our critical temperature criterion for extinction must accordingly be increased to maintain effectively a critical constancy for E/T. These chemical effects are complex and specific, and we will not be able to adequately quantify them. It is sufficient to remember that both velocity (flame stretch) and chemistry (retardant kinetics) can affect extinction. We will only examine the temperature extinction criterion. 

It has been speculated that there is a common origin of the reduced chemical etch rate for (111) oriented silicon substrates and for highly p-type doped substrates. But the electrochemical investigations discussed above indicate that the passivation of highly doped p-type Si can be ascribed to an oxide film already present at OCP, while no such oxide film is observed on (111) silicon below PP. This supports models that ascribe the reduced chemical etch rate on (111) planes to a retarded kinetic for Si surface atoms with three backbonds, present at (111) interfaces [Gil, A12], as discussed in Section 4.1. 

When the amino group is fully deprotonated, the rate of the H-transfer is 1.5 x 10s s4, but also around pH 7 the reaction is still fast, at the ms timescale (for a quantum mechanical study see Rauk et al. 2001). Upon the decay of the amnioal-kyl radicals formed in reaction (35) ammonia as formed in a yield that points to disproportionation as the major process (Zhao et al. 1997). The fact that the ami-noalkyl radical is the thermodynamically favored species does not mean that the repair of DNA radicals by GSH (Chap. 12.11) is not due to its action as a thiol. As with many other examples described in this book, the much faster kinetics that lead to a metastable intermediate (here the formation of the thiyl radical) rather than the thermodynamics as determined by the most stable species (here the aminoalkyl radical) determine the pathway the the reaction. In fact, the C-H BDE of the peptide linkage is lower than the S-H BDE and repair of DNA radicals by peptides, e.g., proteins would be thermodynamically favored over a repair by thiols but this reaction is retarded kinetically (Reid et al. 2003a,b). 

Schulz, H. Bohringer, W. Waller, P. Ousmanov, F. Gas oil deep hydrodesulfurization refractory compounds and retarded kinetics. Catal. Today 1991, 49, 87-97. 

Under neutral conditions, the observed mechanism of hydrolysis cannot be explained by a simple superposition of the retardation kinetics at high pH and intramolecular catalysis at low pH (92). 

Alkaline metal cations at extra-framework sites of the zeolite structure that were coordinated weakly, have the highest catalytic activity with respect to COS formation, in accordance with the process of base catalysis. LTA zeolites with complete alkaline earth cation exchange exhibit the lowest activity due to sufficiently retarded kinetics of COS formation. Nonetheless, even these materials lead the COS formation to its equilibrium after long contact times. 

Purity of Absorption Hydrogen Source Stream The storage capacity of the hydrides are very sensitive to even minute levels of gas-phase impurities. Gas-phase impurities can retard kinetics, reduce storage capacity, or cause irreversible corrosion of the storage alloy matrix. 

Should be pointed out to relatively high values of the sorption of n-CgFj g on Styrosorbs at low P/Pq values and irreversibility of the isotherms in the initial region. The latter may be attributed both to slight swelling of polymers in this sorbate and to retarded kinetics of the sorption of n CgFj g in the micropores present in these seunples. 

Nitration in aqueous solutions of nitric acid Added water retards nitration in concentrated nitric acid without disturbing the kinetic order of the reaction. The rate of nitration of nitrobenzene was depressed sixfold by the addition of 5 % of water, (c. 3 2 mol 1 ), but because of the complexity of the equilibria involving water, which exist in these media, no simple relationship could be found between the concentration of water and its effect on the rate. 

The effects of added species. The rate of nitration of benzene, according to a rate law kinetically of the first order in the concentration of aromatic, was reduced by sodium nitrate, a concentration of io mol 1 of the latter retarding nitration by a factor of about Lithium nitrate... 

In most electron spectroscopic analyses, the kinetic energies of the electrons entering the analyzer are retarded to either a constant energy or by a constant factor. These approaches lead to two modes of operation the constant analyzer energy (CAE) mode and the constant retard ratio (CRR) mode. 

The CRR mode involves retarding the electron kinetic energies to a constant ratio of H /H where H is the energy passed by the analyzer. Thus, the energies are retarded by a constant factor. Spectra acquired in this mode ate less easy to quantify, but small peaks at low kinetic energies ate readily detected. This mode of operation results in spectra of constant relative resolution throughout. The relative resolution is improved in this mode by a factor of E. ... 

Octabromodiphenyl Oxide. Octabromodiphenyl oxide [32536-52-0] (OBDPO) is prepared by bromination of diphenyl oxide. The degree of bromination is controlled either through stoichiometry (34) or through control of the reaction kinetics (35). The melting poiat and the composition of the commercial products vary somewhat. OBDPO is used primarily ia ABS resias where it offers a good balance of physical properties. Poor uv stabiUty is the primary drawback and use ia ABS is being supplanted by other brominated flame retardants, primarily TBBPA. 

In studies of the polymerization kinetics of triaUyl citrate [6299-73-6] the cyclization constant was found to be intermediate between that of diaUyl succinate and DAP (86). Copolymerization reactivity ratios with vinyl monomers have been reported (87). At 60°C with benzoyl peroxide as initiator, triaUyl citrate retards polymerization of styrene, acrylonitrile, vinyl choloride, and vinyl acetate. Properties of polyfunctional aUyl esters are given in Table 7 some of these esters have sharp odors and cause skin irritation. 

High alkalinities of limed juice serve several functions. Foremost is to retard sucrose hydrolysis, one of the oldest reactions in the Uterature of chemical kinetics (6). Sucrose hydrolysis proceeds much more slowly at a moderately high pH than at an even slightly acidic pH. 

Amines can also swell the polymer, lea ding to very rapid reactions. Pyridine, for example, would be a fairly good solvent for a VDC copolymer if it did not attack the polymer chemically. However, when pyridine is part of a solvent mixture that does not dissolve the polymer, pyridine does not penetrate into the polymer phase (108). Studies of single crystals indicate that pyridine removes hydrogen chloride only from the surface. Kinetic studies and product characterizations suggest that the reaction of two units in each chain-fold can easily take place further reaction is greatiy retarded either by the inabiUty of pyridine to diffuse into the crystal or by steric factors. 

Another problem occurs when some fire retardant formulations ate exposed to elevated temperatures (eg, when used as roof tmsses or as roof sheathing) thermal-induced strength reductions can occur in-service. The thermo-chemical factors were discussed by LeVan and Winandy (26), and a kinetic degrade model was developed (27). The treater should be consulted to obtain appropriate in-service modifications for specific fire retardant treatments. 

The reaction mechanisms are quite complicated in these syntheses, the kinetics depending on inter-diffusion rates in neighbouring particles, the formation of transient liquid phases, and in some cases, the vapour transport of a reactant. The presence of the latter can be detected by dre retarding effect of increased pressure in an inert surrounding atmosphere. 

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

In XPS the photoelectrons are retarded to a constant energy, called the pass energy, as they approach the entrance slit. If this were not done, Eq. (2.5) shows that to achieve an absolute resolution of 1 eV at the maximum kinetic energy of approximately 1500 eV (using A1 Ka radiation), and with a slit width of 2 mm, would require an analyzer with an average radius of about 300 cm, which is impracticable. Pass energies are selected in the range 20-100 eV for XPS, which enables the analyzer to be built with a radius of 10-15 cm. 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

Nitroalkanes show a related relationship between kinetic acidity and thermodynamic acidity. Additional alkyl substituents on nitromethane retard the rate of proton removal although the equilibrium is more favorable for the more highly substituted derivatives. The alkyl groups have a strong stabilizing effect on the nitronate ion, but unfavorable steric effects are dominant at the transition state for proton removal. As a result, kinetic and thermodynamic acidity show opposite responses to alkyl substitution. 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

From the standpoint of geometrical considerations, the major difference is in the far greater steric requirements of the nitro group. This could result in either primary or secondary steric effects. Nevertheless, primary steric effects do not seem to be necessarily distinguishable by direct kinetic comparison. A classic example is the puzzling similarity of the activation parameters of 2-chloropyrimidine and 2,6-dinitrochlorobenzene (reaction with piperidine in ethanol), which has been described by Chapman and Rees as fortuitous. However, that nitro groups do cause (retarding) primary steric effects has been neatly shown at peri positions in the reaction with alkoxides (see Section IV,C, l,c). 

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